Axial piston pumps can be used in aeroengine fuel control systems, e.g. to provide pressurised fuel to actuate auxiliary engine devices such as variable vanes and bleed valves, and to provide additional fuel supply to the combustor at high burnt flow engine operating conditions. An example of an axial piston pump in such a control system is described in US 2015/0192075. In this system, a gear pump is used to supply high pressure fuel to the combustor, with an axial piston pump providing extra capacity for the auxiliary devises. During periods of high burnt flow demand, the axial piston pump can be used to augment the gear pump.
An axial piston pump is a positive displacement pump containing one or more pistons within a piston housing or rotor. The pump operates by rotation of the rotor and hence the piston(s) relative to a swashplate, also known as a cam plate, which is generally a flat disc angled relative to the piston(s) and the axis of rotation. The swashplate functions as a cam to transform rotational movement into reciprocal movement of the piston(s). Between the rotor and inlet and outlet ports of the pump an interface exists. Conventionally, two hardened, flat, high polished plates are forced together at this interface to form a seal between the rotor and the ports, one of the plates rotating with the rotor and the other being fixed. To achieve this seal, the pistons are carefully balanced against the hydrodynamic pressure of a lubricating fluid at the interface over a set of running conditions.
However, should the pump be taken outside of its normal operating window, there is a risk that a pressure distribution generated in the lubricating film (generally a few microns thick) between the rotating and fixed plates (as a result of their relative movement) will be sufficient to force the two sealing plates apart and, for example, push the rotor away from the inlet and outlet ports. Any gap between the surfaces could allow expelled high pressure fluid intended for the pump outlet port to leak back into the pump inlet port, to the detriment of pump efficiency. Further, as the pressure in the region surrounding the outlet port in the fixed plate is generally greater than that surrounding the inlet port, the resultant pressure distribution may generate a moment which can induce a tilt in the rotating components and/or the fixed plate, if they are not adequately supported.
Similar issues also pertain to axial piston motors, which are essentially the same device operated in reverse so that high pressure fluid drives reciprocal movement of the pistons, which in turn produces rotation of the rotor.